1. Field of the Invention
The present invention relates to an air-fuel ratio control technique for an internal combustion engine.
2. Description of the Background Art
Generally, an exhaust path of an internal combustion engine is provided with a three-way catalyst for simultaneously cleaning HC, CO, and NOx contained in the exhaust gas. With this catalyst, a high conversion ratio is obtained in the vicinity of a predetermined air-fuel ratio (theoretical air-fuel ratio) for all of HC, CO, and NOx. For this reason, an oxygen concentration sensor is usually provided upstream of the catalyst so that an air-fuel ratio that is identified from its detection result is controlled to become close to the theoretical air-fuel ratio.
However, the oxygen concentration sensor provided upstream of the catalyst causes characteristic fluctuations (errors) since it is exposed to high exhaust temperatures; in view of this, there has been proposed a control device for an internal combustion engine in which an oxygen concentration sensor is also provided downstream of the catalyst so that errors can be corrected according to output values from the oxygen concentration sensor downstream of the catalyst (see, for example, Japanese Patent Application Laid-Open No. 6-42387 (1994)). In other words, in the device proposed in the foregoing publication, the oxygen concentration sensors are disposed both upstream and downstream of the catalyst to control the air-fuel ratio so that the atmosphere in the catalyst is maintained in the vicinity of the theoretical air-fuel ratio.
In the device proposed in the foregoing publication, a proportional operation and an integral operation are performed based on the result of comparison between an output from the oxygen concentration sensor and a target value concerning the downstream side of the catalyst, whereby the target value for the upstream side of the catalyst is corrected, and a fuel supply amount to an internal combustion engine is controlled by using a proportional operation and an integral operation so that the output of an oxygen concentration sensor and a target value match each other concerning the upstream side of the catalyst. Thus, it is possible to prevent tracking delays in the controlling and excessive corrections.
Further, in the device proposed in the foregoing publication, when the internal combustion engine enters a transient state due to a sudden closure of the throttle valve or the like, it stops the integral operation concerning the downstream side of the catalyst from the time of switching to the transient state to the lapse of a predetermined period. At this time, the integral value obtained by the integral operation is maintained at a value obtained immediately before entering the transient state, thereby suppressing the excessive correction of the target value of the air-fuel ratio regarding the upstream, which is caused when leaving the transient state. That is, it is possible to suppress the deviation of the air-fuel ratio caused by the transient state.
The above-mentioned catalyst provided in the exhaust path of the internal combustion engine has a capability of storing oxygen according to the oxygen concentration in the exhaust gas (oxygen storage capability) in order to compensate the temporary deviation of the air-fuel ratio in the internal combustion engine from the theoretical air-fuel ratio. Because of the oxygen storage capability, if the air-fuel ratio is leaner than the theoretical air-fuel ratio, the catalyst takes in the oxygen in the exhaust gas and stores it, whereas if the air-fuel ratio is richer than the theoretical air-fuel ratio, the catalyst discharges the oxygen stored therein. As a result, the atmosphere in the catalytic converter is maintained in the vicinity of the theoretical air-fuel ratio. However, when the fluctuation of the air-fuel ratio is great in the transient state and the amount of oxygen storage reaches zero or the upper limit value, the atmosphere in the catalyst is no longer maintained in the vicinity of the theoretical air-fuel ratio, deviating greatly from the theoretical air-fuel ratio.
As described above, three-way catalysts show high conversion ratios for all of HC, CO, and NOx in exhaust gases in the vicinity of the theoretical air-fuel ratio, and the conversion ratios become highest when the amount of oxygen storage is at an appropriate amount, about half of the upper limit value. In addition, the amount of oxygen storage of a catalyst can be detected from a very small variation of the air-fuel ratio in the downstream of the catalyst, which is in the vicinity of the theoretical air-fuel ratio. Accordingly, by controlling the air-fuel ratio of the upstream side of the catalyst according to values detected by the oxygen concentration sensor in the downstream side of the catalyst, the amount of oxygen storage can be controlled to be an appropriate amount and the conversion ratio of the catalyst can be kept high.
Nevertheless, the function of oxygen storage in catalyst serves as a cause of response delays in the air-fuel ratio control. Specifically, even when the air-fuel ratio of the upstream of the catalyst is changed to be richer or leaner by a feedback control, the air-fuel ratio of the downstream of the catalyst does not correspond immediately but changes after the amount of oxygen storage in the catalyst has changed.
Thus, if the integral operation concerning the downstream of the catalyst is restarted after the lapse of a certain time from a time of transition to a state in which the fuel supply to the internal combustion engine is stopped (a fuel cutoff state) without taking the behavior of the amount of oxygen storage into consideration, as the device proposed in the foregoing publication, problems arise such as malfunctions (excessive corrections) in the feedback control and impairing of its primary function. As a result, the air-fuel ratio after the fuel cutoff tends to deviate from the theoretical air-fuel ratio, leading to deterioration of emissions or the like.